1. Field of the Invention
The present invention relates to a multi-beam scanning device constituting exposure means for use in an electronic photographing device in which an image is formed by scanning an image carrier with a plurality of beams.
2. Description of the Related Art
A beam scanning device as exposure means in an electronic photographing device has been heretofore used in writing an image in an electronic photograph process, and mounted on an output device, such as a laser beam printer, of a computer or on a laser facsimile of a facsimile machine and the like. Recently, in order to achieve a high speed operation and a high resolution, there has been increasingly a demand for a scanning optical system by a plurality of beams.
A conventional multi-beam scanning device for emitting a multiplicity of beams will be described hereinafter. FIGS. 5A and 5B show a perspective view of a two-beam scanning device as an example of the conventional multi-beam scanning device. FIGS. 6A and 6B are views showing a state, in which a pitch for two beams is adjusted in a sub-scanning direction.
In FIGS. 5A and 5B, the reference numerals 1a, 1b denote first and second light sources; 2a, 2b denote collimator lenses for transforming beams from the respective light sources into substantially parallel beams; 3a, 3b denote prisms for adjusting the pitch for the beams emitted from the first and second light sources 1a, 1b in the sub-scanning direction; 4a, 4b denote adjustment mechanisms for turning the prisms 3a, 3b; 5 denotes a beam splitter for aligning optical axes of the beams emitted from the respective light sources; 6 denotes a cylindrical lens for converging the beams emitted from the beam splitter 5 in the sub-scanning direction; 7 denotes a deflector provided with a deflecting surface in the vicinity of a converged point of the cylindrical lens 6 for simultaneously deflecting two beams; 8 denotes a scanning lens system for converging the beams deflected by the deflector 7 onto a surface being scanned to perform scanning; 9 denotes a synchronization detector provided in a non-image area on a beam scanning starting side for synchronizing the scanning beams; 10 denotes a sensor section for detecting the pitch interval of two beams in the sub-scanning direction; 11 denotes a divided sensor A mounted on the sensor section 10; 12 denotes a divided sensor B; 13, 14, 15, 16 denote light receiving surfaces of the two divided sensors; 17 denotes a control circuit which turns the prisms 3a, 3b based on a signal from the sensor section 10 to control the pitch of the beams in the sub-scanning direction; 18 denotes a surface being scanned; 19 denotes a mirror for directing the beams to the surface being scanned 18; 20 denotes a housing; 21 denotes the beam from the light source 1a; 22 denotes the beam from the light source 1b; 23 denotes a drive circuit for the light source 1a; 24 denotes a drive circuit for the light source 1b; and 25 denotes a deflecting surface.
The operation of the two-beam scanning device constituted as described above will be described hereinafter.
In FIGS. 5A and 5B, the beams emitted from the first and second light sources 1a, 1b are adjusted by the collimator lenses 2a, 2b to form substantially parallel beams. The prisms 3a, 3b for adjusting a pitch in the sub-scanning direction to a predetermined interval are arranged on optical paths of the respective beams. The prism 3a is used for the changing of the optical path of the beam 21 from the first light source 1a, while the prism 3b is used for the changing of the optical path of the beam 22 from the second light source 1b. The beam splitter 5 coincides the optical axes of the beams 21, 22 transmitted through the prisms 3a, 3b with each other, and the beams are then converged in the sub-scanning direction by the cylindrical lens 6. This is for the purpose of placing the deflecting surfaces 25 and the surface being scanned 18 in an optically conjugate relationship to mitigate an influence of respective surface inclinations which the plurality of deflecting surfaces 25 of the deflector 7 involve.
Since the deflecting surfaces 25 and the surface being scanned 18 have the conjugate relationship in the sub-scanning direction as described above, a pitch for the two beams on the deflecting surface 25 is projected onto the surface being scanned 18 at the magnification of the scanning lens system 8 in the sub-scanning direction. More specifically, the pitch for the two beams on the surface being scanned 18 can be corrected by controlling the pitch for the two beams on the deflecting surface 25.
In order to control the pitch, the detecting sensor section 10 is disposed in the sub-scanning position of each beam. The sensor section 10 is positioned in such a manner that the beam emitted in a direction other than a direction extending from the beam splitter 5 toward the deflecting surface 25 is incident in a position which is optically equivalent to a position where the deflecting surface 25 is disposed.
The sensor section 10 is composed of the divided sensor All for positional adjustment of the beam 21 from the first light source 1a and the divided sensor B12 for positional adjustment of the beam 22 from the second light source 1b, and the respective beams are made to register with boundaries of the two divided sensors (a middle of the light receiving surfaces 13, 14 or a middle of the light receiving surfaces 15, 16).
The two divided sensors A11 and B12 are fixed to be offset from each other by a predetermined amount (corresponding to a predetermined pitch on the surface being scanned 18) in the sub-scanning direction. In order that the beam 21 emitted from the first light source 1a be made the divided sensor A11 of the sensor section 10 to register with a predetermined position, the drive circuit 23 allows only the first light source 1a to emit light to radiate the beam 21 to the divided sensor A11. In this case, a signal of the control circuit 17 allows the prism 3a to turn to change an optical path of the beam from the first light source 1a in the sub-scanning direction so that an output from the light receiving surface 13 and an output from the light receiving surface 14 have the same level in terms of intensity of irradiation.
FIG. 6A to 6C are views showing a relationship between beam positions and outputs. For example, when the beam 21 is on the side of the light receiving surface 13 as shown in FIG. 6A, an output level from the light receiving surface 13 is high as compared with that from the light receiving surface 14. On the contrary, when the beam is on the side of the light receiving surface 14 as shown in FIG. 6C, an output level of the light receiving surface 14 is high as compared with that of the light receiving surface 13. Therefore, a difference between the output levels of the light receiving surfaces 13, 14 makes it possible to judge the current beam position and a direction in which the beam should be moved, and the control circuit 17 allows the prism 3a to be turned while controlling the adjustment mechanism 4a.
FIG. 7 shows optical paths in the conventional two-beam scanning device. As shown in FIG. 7, the optical path is changed by the prism 3a such that an angle of the prism 3a is shifted toward the sub-scanning direction to change a position of the beam on the deflecting surface 25 in the sub-scanning direction. Subsequently, as shown in FIG. 6B, at the time when the light receiving surfaces 13 and 14 become the same in output level, turning of the prism 3a is stopped. At this time, a center of the beam 21 is made to register with a boundary of the light receiving surfaces 13, 14.
The similar operation to that described above enables making the beam 22 of the second light source 1b registering with the boundary of the light receiving surfaces 15, 16. Since the boundaries of the divided sensor A11 having the light receiving surfaces 13, 14 and of the divided sensor B12 having the light receiving surfaces 15, 16 are offset a predetermined amount in the sub-scanning direction (corresponding to the predetermined pitch on the surface being scanned 18), the aforementioned adjustment enables the surface being scanned 18 to be scanned with two beams at the predetermined pitch. The adjustment described above is performed before the surface being scanned 18 is scanned with the beams, and the pitch for the two beams is adjusted to the predetermined value.
In practice, when the surface being scanned 18 is to be scanned with the beams, the positions of the two divided sensors A11, B12 are adjusted beforehand at the time of assembly/adjustment. An operation is performed, in which the beams are made to register with predetermined positions of two divided sensors A11 and B12, which have been subjected to the positional adjustment.
Two beams 21, 22 having been adjusted to the predetermined pitch are incident upon the deflector 7 to be converged onto the surface being scanned 18 by the scanning lens system 8 for scanning. Generally, the optical path in the scanning lens system 8 is directed to the surface being scanned 18 by the mirror 19 to make the optical system compact. Two beams, which are made by the deflector 7 to scan, are subjected to synchronization in a main scanning direction by the synchronization detector 9 which is provided in a position this side of a position where the image area is scanned, and irradiation corresponding to the image data is effected with a predetermined timing by the drive circuits 23, 24 of the respective light sources 1a, 1b. Since the surface being scanned 18 is moved in the sub-scanning direction, an image can be formed on the surface being scanned 18 by the two-dimensional irradiation.
FIGS. 8A and 8B are diagrammatic views showing images formed. When the pitch for the beams is adjusted to the predetermined interval as described above, a uniform image can be obtained as shown in FIG. 8A. On the other hand, FIG. 8B shows an image when the beam pitch gets out of position.
As described above, since two beams form an image on the surface being scanned 18, a high-speed or high-resolution beam scanning device can be obtained.
With the aforementioned conventional arrangement, however, the prisms 3a, 3b are turned to change angles, at which beams are incident upon the cylindrical lens 6, thereby adjusting a pitch for two beams. Thus, the two beams are transmitted through the scanning lens system 8 at different incident angles (sub-scanning directions), so that planes, through which the two beams are transmitted, are different in the scanning lens system 8, which is liable to generate curved image planes and differences in scanning speed during the scanning of the surface being scanned 18.
Moreover, since the beam pitch is determined by a predetermined offset of the two divided sensors A11, B12 in the sub-scanning direction, a high mount accuracy is required with respect to the two divided sensors A11, B12, which makes a mounting operation of the sensors difficult. Furthermore, in order to set several kinds of pitches for the two beams, it is necessary to add a corresponding number of divided sensors, which increases cost and disadvantageously makes it difficult to secure a space for the sensor section.
An object of the invention is to provide a multi-beam scanning device which prevents generation of differences in the scanning speed, curved image planes and the like for a plurality of beams and facilitates pitch adjustment of the plurality of beams in the sub-scanning direction.